Fuel cells for combining hydrogen and oxygen to produce electricity are well known. A known class of fuel cells includes a solid-oxide electrolyte layer through which oxygen anions migrate; such fuel cells are referred to in the art as “solid-oxide” fuel cells (SOFCs).
In some applications, for example, as an auxiliary power unit (APU) for a transportation application, an SOFC is preferably fueled by “reformate” gas, which is the effluent from a catalytic liquid or gaseous hydrocarbon oxidizing reformer, also referred to herein as “fuel gas”. Reformate typically includes amounts of carbon monoxide (CO) as fuel in addition to molecular hydrogen.
A complete fuel cell stack assembly includes fuel cell subassemblies and a plurality of components known in the art as interconnects, which electrically connect the individual fuel cell subassemblies in series. Typically, the interconnects include a conductive foam or weave disposed in the fuel gas and air flow spaces adjacent the anodes and cathodes of the subassemblies.
In the prior art, a fuel cell stack is assembled typically by first laying up each of the fuel cell subassemblies in a jig, forming a plurality of repetitive modular fuel cell units known in the art, and referred to herein, as “cassettes”. Typically, a fuel cell cassette comprises a ceramic solid-oxide electrolyte layer and a cathode layer coated onto a relatively thick, structurally significant anode element. In such a prior art assembly, each of the cassettes becomes a structural and load-bearing element of the stack.
At the elevated operating temperatures of an SOFC stack, typically in the range of about 700° C. to about 1000° C., most of the components of a cassette have very little inherent mechanical strength and would collapse if not for internal spacer rings disposed within each cassette around the anode fuel gas openings, collectively comprising supply and exhaust “chimneys” within a stack. Prior art spacer rings are fabricated so that they form a solid column of metal having radial openings to allow the anode fuel gas to flow into and out of the cassette. The assembly load of a stack thus is carried through the spacer rings.
A prior art spacer ring is a sheet metal part which is stamped and formed to achieve the desired geometry. This spacer ring is difficult to form, resulting in a part that is relatively expensive even with production tooling in high volumes. Further, each cassette requires a plurality of spacer rings (typically 8), and each ring must be tack welded into place to the cassette shell, accurately and firmly, prior to cassette assembly, adding further positioning and attachment cost and complexity to the assembly operation.
What is needed in the art is an improved spacer ring that is less expensive to manufacture and less expensive to install into a cassette during assembly thereof.
It is a principal object of the present invention to reduce the cost, difficulty, and complexity of mass-manufacturing fuel cell stack assemblies.